It is well known that terephthalic acid is suitable for the production of polyester and copolymer thereof in textile fibre, and this polymer and copolymer thereof can be made by condensation of terephthalic acid and dihydric alcohol. Crude terephthalic acid can be purified by hydrogenation in the presence of a suitable catalyst, and hydrogenation is the easiest route for removal of 4-carboxylbenzaldehyde (4-CBA) impurity from the crude terephthalic acid.
Catalyst comprising a Group VIII metal of the Periodic Table of Elements supported on an inert support is useable in various hydrogenation reactions. Such a catalyst is typically prepared by impregnating a support material with a solution of a compound of Group VIII metal and then reducing the impregnated compound to the metal. Similar to other supported catalysts, the activity and selectivity of a catalyst comprising a Group VIII metal supported on a support depend on many factors, such as the amount of active metal component supported on the support, the type of support, and the process for preparing the catalyst, i.e., the process for depositing or dispersing the Group VIII metal on the support.
Pa/C catalyst is a common hydrogenation catalyst, and its studies and improvements always focus on the distribution state of metallic palladium as active component on support, because this greatly affects the property of catalyst.
As to a catalyst comprising a noble metal as active component supported on a granular or shaped activated carbon support, the active component is generally distributed on the surface of support, and the active component distributed in the support is out of function. Initially, the active component is distributed deeply in support, for example, U.S. Pat. No. 4,791,226 discloses the distribution of palladium is within a range of 0-500 μm from the surface of support. U.S. Pat. No. 4,728,630 mentions that metal crystallites are mainly distributed in a range from the surface of support to a depth of at least about 5 μm, and said depth is preferably from about 10 to 20 μm. U.S. Pat. No. 4,467,110 mentions that the penetration depth of active component in a porous support ranges from about 70 to about 150 μm from the surface of support. CN1,283,521A mentions that the active component of catalyst is distributed in a layer from the outer surface of support to a depth of 1%-30% of the radius thereof. U.S. Pat. No. 6,066,589 mentions that less than 50 wt % of palladium is comprised in a surface layer having a thickness of up to 50 μm, and the remainder is distributed in an inner layer having a depth from about 50 to about 400 μm. These studies about the distribution of palladium active component in support indicate that the property of catalyst is improved in some extent.
As to the crystallite size of metallic palladium active component, the main common understanding in the recent studies is that the smaller the better, i.e., the crystallite size is less than 35 Å, and the smaller crystallite size, the more uniform distribution of metallic palladium active component on support. For example, U.S. Pat. Nos. 4,415,479, 4,421,676 and 4,791,226 all mention that the palladium crystallite size in catalyst are less than 35 Å. U.S. Pat. No. 4,394,299 and U.S. Pat. No. 4,791,226 further teach that when the palladium crystallite size in catalyst are greater than 35 Å, the surface area of palladium decreases, and the activity of Pd/C catalyst in the hydrogenation of 4-CBA decreases as well.
In the meantime, it is by no means that the higher the palladium content on the surface of support is the better, because when a large amount of metallic palladium concentrates on the surface of support, the formation of palladium cluster will be facilitated and the crystallite size of metallic palladium will increase, which causes the decrease of the surface area of palladium and the decrease of catalytic activity. Further, when a large amount of metallic palladium concentrates on the surface of support, the loss of metallic palladium caused by the erosion of stuffs will increase during the hydrogenation of terephthalic acid. If the palladium content on the surface of support is at a relatively low level, the surface area of palladium and the catalytic activity will decrease as well. Thus, it is very important to control the palladium content on the surface of support.
Currently, as to a process for preparing a catalyst comprising a Group VIII metal supported on an activated carbon support, reports mainly focus on the following aspects: (1) mixing by a mechanical stirrer, for example, U.S. Pat. Nos. 4,791,226, 4,415,479, 4,394,299, 4,421,676, etc. all stir with a paddle, located in a solvent layer above the carbon in order to uniformly adsorb a palladium salt solution on an activated carbon support; and (2) mixing by a rotating reactor, for example, U.S. Pat. No. 4,728,630, CN1,283,521A, etc. use a cylindrical vessel to uniformly mix a palladium salt solution and an activated carbon support. Both of these two manners can uniformly mix solution and support.
The mechanical stirrer is generally suitable for a noble metal catalyst supported on a powdery carbon support. As to a granular or shaped carbon support, when the amount of support is at a relatively low level, the support can readily contact with solution sufficiently and is hardly broken by the stirrer, but when the amount of support is at a relatively high level, the support cannot sufficiently contact with solution under mechanical stirring. Under gentle stirring condition, there are some corners without stirring, where the support is actually impregnated with solution under static state; further, the activated carbon support generally is basic, and such a strong basic will facilitate the formation of noble metal hydroxide deposit, which will cause the loss of noble metal. Under vigorous stirring condition, although there is no corner without stirring, the light activated carbon moves with the eddy caused by the stirring and is damaged extremely by the stirrer. In particular, as to the hydrogenation catalyst for purifying terephthalic acid, the metallic palladium in shell distribution on the surface of support may be brushed off by the vigorous stirring, so that the obtained catalyst may have a reduced surface palladium content and a reduced activity.
The rotating reactor generally employs a non-excessive adsorption technique, i.e., the quantity of the adsorbed solution is not more than the saturated adsorption capacity of support. With the use of such a technique, said reactor generally is suitable for preparing a catalyst comprising an active component distributed on the surface of support, such as a Pd/C catalyst for the hydrogenation and purification of terephthalic acid. Said reactor uses a nozzle to atomize the solution, and the support rotates with the reactor in order to form uniform adsorption. However, it is difficult to sufficiently contact the rotating activated carbon support with the atomized solution, which facilitates the formation of nonuniform adsorption. Further, due to the non-excessive adsorption of the activated carbon, the directly friction among particles of activated carbon will cause the formation of fine powder of catalyst and the loss of noble metal.
It can be seen that people still want to develop a catalyst comprising a distinct metallic palladium distribution on a support, and a corresponding process for preparing said catalyst via continuous studying.